Optical fiber amplifiers have quickly found use in optical telecommunications networks, particularly those that rely on optical fibers to waveguide light over very long distances. Although modern silica optical fibers exhibit very low loss in the 1.3 and 1.5 .mu.m windows, they are lossy to some extent and the loss accumulates over long distances. Optical fiber amplifiers allow the optical signal to be amplified without the need to convert the optical signal to an electrical signal and then regenerate the optical signal.
Erbium-doped fiber amplifiers, which are now well developed, amplify in a wavelength band between about 1.53 and 1.57 .mu.m, well suited for silica fibers. However, even within this narrow bandwidth, erbium-doped amplifiers exhibit a distinct spectral variation in gain, as illustrated by gain spectrum 10 of FIG. 1. The lack of a flat gain spectrum over a wide bandwidth causes several problems. Extremely short optical pulses have a relatively wide power spectrum and are not accurately amplified if the gain spectrum is not flat. The lack of flat gain causes a lack of gain stability because the features of the gain spectrum depend upon factors, such as temperature and pumping levels, which are difficult to control.
Wavelength division multiplexing (WDM) is being developed to exploit the huge bandwidth of optical fiber without the need for extremely fast transmitters and detector. In WDM, the fiber receives data-modulated optical signals from several optical transmitters, each of which uses a different optical carrier frequency. With a channel spacing of 4 nm, a single erbium-doped fiber amplifier can simultaneously amplify about 10 WDM channels. However, if several amplifiers are cascaded, as would be required for a transoceanic cable, for example, the total gain spectrum of all the amplifiers becomes even less flat, and the optical carriers at the gain peaks may saturate while those on the skirts or valleys are insufficiently amplified. The same problem may occur in a broadcast system, where loss is due to splitting of the signal into many separate channels.
Past efforts to flatten the gain spectrum have mostly relied on passive or active filtering of the high-gain features of the gain spectrum, which requires close matching of the particular amplifier and filter and which must account for any temporal variation in the gain spectrum. Desurvire et al. has disclosed an electronic feedback control of the gain in "Dynamic Gain Compensation in Saturated Erbium-Doped Fiber Amplifiers", IEEE Photonics Technology Letters, volume 3, 1991, pp. 453-455. Their method stabilizes the gain and reduces crosstalk. Zirngibl has disclosed an all-optical version of the feedback control in "Gain control in erbium-doped fibre amplifiers by an all-optical feedback loop," Electronics Letters, volume 27, 1991, pp. 560-561. He sets up a ring laser through the amplifier that oscillates at a frequency out of the signal band. He describes the radiation in the feedback loop as traversing the amplifier in the same direction as the signal being amplified. Moores has developed the theory of saturating inhomogeneously broadened amplifiers for purposes of gain stabilization in "Ultra-Long Distance Wavelength-Division-Multiplexed Soliton Transmission Using Inhomogeneously Broadened Fiber Amplifiers," Journal of Lightwave Technology, volume 10, 1992, pp. 482-487.